Pseudogas neutron detector

ABSTRACT

Embodiments of the present disclosure include a system for detecting neutrons with a housing, a gas chamber at least partially defined by the housing, an anode extending through at least a portion of the gas chamber, and a pseudogas arranged within the gas chamber. The pseudogas comprises a mixture of gas and suspended solid particles that contain an element with a high cross-section for thermal neutron capture.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/041,379 filed Jul. 20, 2018 titled “PSEUDOGAS NEUTRONDETECTOR,” now U.S. Pat. No. 10,502,849, issued Dec. 10, 2019, the fulldisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND 1. Field of the Invention

The present disclosure relates to downhole measurement devices. Moreparticularly, the present disclosure relates to neutron detectors thatmay be used in downhole environments.

2. Description of Related Art

During oil and gas operations, various measurements may be acquireddownhole in order to evaluate one or more formation properties. Incertain situations, nuclear interrogation techniques may be useddownhole where a radiation source is emitted into the formation andsubsequent radiation (e.g., backscatter, prompt gamma-ray, neutrons,etc.) is measured via a detector located on the tool string. For neutrondetection, gas filled helium (He) detectors may be used. Often, the Hein these detectors is Helium-3, which may be expensive or difficult toobtain. Alternative neutron detectors are also expensive, toxic, or maybe unsuitable for downhole environments.

SUMMARY

Applicant recognized the problems noted above herein and conceived anddeveloped embodiments of systems and methods, according to the presentdisclosure, for neutron detectors.

In an embodiment, a system for detecting neutrons includes a housing, agas chamber at least partially defined by the housing, an anodeextending through at least a portion of the gas chamber, and a pseudogasarranged within the gas chamber, wherein the pseudogas comprises amixture of a gas and solid particles.

In an embodiment, a method for forming a neutron detector includesforming a gas chamber, determining a gas to particle ratio for apseudogas, filling the gas chamber with the pseudogas, and sealing thegas chamber.

In an embodiment, a system for detecting neutrons includes a housingforming a cathode of a proportional gas counter, a gas chamber formed atleast partially by the housing, and an anode extending partially throughthe gas chamber. The system also includes a pseudogas formed from acombination of dense gas and Boron-containing particles positionedwithin the gas chamber, wherein the Boron-containing particles arearranged to capture incoming neutrons and increase a current at theanode via the production of charged particles from neutron capture.

In an embodiment, a system for detecting neutrons includes a housingforming a cathode of a proportional gas counter, a gas chamber formed atleast partially by the housing, and an anode extending partially throughthe gas chamber. The system also includes a pseudogas formed from acombination of dense gas and Lithium-containing particles positionedwithin the gas chamber, wherein the Boron-containing particles arearranged to capture incoming neutrons and increase a current at theanode via the production of charged particles from neutron capture.

In an embodiment, a system for detecting neutrons includes a housingforming a cathode of a proportional gas counter, a gas chamber formed atleast partially by the housing, and an anode extending partially throughthe gas chamber. The system also includes a pseudogas formed from acombination of dense gas and Uranium-containing particles positionedwithin the gas chamber, wherein the Boron-containing particles arearranged to capture incoming neutrons and increase a current at theanode via the production of charged particles from neutron capture.

In an embodiment, a system for detecting neutrons includes a housing, agas chamber at least partially defined by the housing, an anodeextending through at least a portion of the gas chamber, and a pseudogasarranged within the gas chamber. The pseudogas comprises a mixture ofgas and solid particles. In an embodiment, the solid particles arehollow and the pressure of the gas is increased to achieve neutralbuoyancy of the hollow particles as they float in the gas.

In another embodiment, a system for detecting neutrons includes ahousing, a gas chamber at least partially defined by the housing, and ananode extending through at least a portion of the gas chamber. Thesystem also includes a pseudogas arranged within the gas chamber,wherein the pseudogas comprises a mixture of a gas and solid particles,the solid particles containing an element that generates a chargedparticle after absorbing a thermal neutron.

In another embodiment, a method for forming a neutron detector includesforming a gas chamber, determining a gas to particle ratio for apseudogas, filling the gas chamber with the pseudogas, and sealing thegas chamber. In another embodiment, the particles are nanoparticleswhich can stay afloat for days or weeks in a gas (very long settlingtimes that depend on particle size and gas density) just from Brownianmotion alone without the need for neutral buoyancy.

In an embodiment, a system for detecting neutrons includes a housingforming a cathode of a proportional gas counter. The system alsoincludes a gas chamber formed at least partially by the housing. Thesystem further includes an anode extending partially through the gaschamber. The system also includes a pseudogas formed from a combinationof a dense gas and particles having high thermal neutron cross sectionspositioned within the gas chamber, wherein the particles aresubstantially uniformly distributed within the gas and they captureincoming thermal neutrons and thereby increase a current at the anodevia the production of charged particles from neutron capture. For thisdisclosure, a dense gas is a gas or a supercritical fluid whose densitycan be increased with pressure at room temperature so as to match theparticle density. In various embodiments, the dense gas is nontoxic andchemically nonreactive, such as the inert gases, Xe or Ar, whosecritical temperatures are below room temperature so that they nevercondense to a liquid at any pressure, no matter how high, whenever thattemperature is room temperature or above. Although there are other densegases, such Sulfur Hexafluoride (critical temperature 45.6 C), SF₆ will,at 343 psi, condense to a liquid at 25 C, whose liquid density thenchanges very little with any further increases in pressure making ithard to match the particle density. The isotope, He-3, has only 0.00014%natural abundance, which is one factor that makes it expensive, and ithas a thermal neutron cross section of 5333 barns. In this disclosure,isotopes, which may be used in the solid particles include B-10 (19.9%abundance, 3835 barns), Eu-151 (47.8% abundance, 9100 barns), Cd-113(12.22% abundance, 20600 barns), Sm-149 (13.9% abundance, 42080 barns),Gd-155 (14.8% abundance, 61100 barns), and Gd-157 (15.7% abundance,259000 barns), Li-6 (7.5% abundance, 936 barns), U-235 (0.72% abundance,690 barns). Boron's low atomic weight permits lower density particleseither as elemental boron or, in various embodiments, as chemicallybonded to another light element such as carbon, nitrogen, or oxygenthereby making lightweight particles for which it is easier to achieveneutral buoyancy (e.g., substantially infinite settling time) in apressurized gas, especially if the particle is hollow. Another advantageof Boron is that the energy released when it captures a neutron is high(2310 keV). It exceeds the 765 keV released when He-3 captures a neutronand it far exceeds the 105 keV released when Gd captures a neutron.Also, B-10 has very low sensitivity to interfering gamma rays. That iswhy Boron Tetrafluoride (BF₃) gas neutron detectors are a commonalternative to He-3 gas neutron detectors. Unfortunately, BF₃ is bothtoxic and corrosive and B-10 enriched BF₃ is expensive. The number ofB-10 nuclei per unit volume of 100% B-10 enriched BF₃ at the typical 1.0atmosphere of BF₃ gas pressure that is used can be a factor of 1900times less than that achieved by embodiments of the present disclosure'snatural-abundance, boron-containing-particle pseudogas at the maximumparticle concentration.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing aspects, features, and advantages of the presentdisclosure will be further appreciated when considered with reference tothe following description of embodiments and accompanying drawings. Indescribing the embodiments of the disclosure illustrated in the appendeddrawings, specific terminology will be used for the sake of clarity.However, the disclosure is not intended to be limited to the specificterms used, and it is to be understood that each specific term includesequivalents that operate in a similar manner to accomplish a similarpurpose.

FIG. 1 is a schematic side view of an embodiment of a drilling system,in accordance with embodiments of the present disclosure;

FIG. 2 is a schematic cross-sectional view of an embodiment of a neutrondetector, in accordance with embodiments of the present disclosure;

FIG. 3 is a schematic cross-sectional view of an embodiment of a gaschamber with a pseudogas, in accordance with embodiments of the presentdisclosure;

FIG. 4 is a schematic cross-sectional view of an embodiment of a gaschamber with a pseudogas, in accordance with embodiments of the presentdisclosure;

FIG. 5 is a flow chart of an embodiment of a method for forming aneutron detector with a pseudogas, in accordance with embodiments of thepresent disclosure; and

FIG. 6 is a flow chart of an embodiment of a method for performingneutron detection, in accordance with embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The foregoing aspects, features, and advantages of the presentdisclosure will be further appreciated when considered with reference tothe following description of embodiments and accompanying drawings. Indescribing the embodiments of the disclosure illustrated in the appendeddrawings, specific terminology will be used for the sake of clarity.However, the disclosure is not intended to be limited to the specificterms used, and it is to be understood that each specific term includesequivalents that operate in a similar manner to accomplish a similarpurpose.

When introducing elements of various embodiments of the presentdisclosure, the articles “a”, “an”, “the”, and “said” are intended tomean that there are one or more of the elements. The terms “comprising”,“including”, and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements. Anyexamples of operating parameters and/or environmental conditions are notexclusive of other parameters/conditions of the disclosed embodiments.Additionally, it should be understood that references to “oneembodiment”, “an embodiment”, “certain embodiments”, or “otherembodiments” of the present disclosure are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features. Furthermore, reference to termssuch as “above”, “below”, “upper”, “lower”, “side”, “front”, “back”, orother terms regarding orientation or direction are made with referenceto the illustrated embodiments and are not intended to be limiting orexclude other orientations or directions.

Embodiments of the present disclosure include systems and methods fordetecting radiation in a downhole environment, such as neutrons within aproportional gas counter. In various embodiments, a neutron detectorincludes a gas chamber that is filled with a pseudogas containing acombination of gas and solid particles, such as substantially hollowparticles. In various embodiments, the particles contain Boron and thesurrounding gas is Xenon, an inert gas whose high atomic weight permitssignificant mass density at moderate pressures. In various embodiments,a mass density of the Boron-containing particles may be substantiallyequal to the Xenon mass density so that the Boron particles cansubstantially “float” or be suspended within the Xenon gas. As a result,neutrons entering the gas chamber may interact with the substantiallyuniformly-distributed Boron at a variety of different locations. Invarious embodiments, the Boron utilized to produce the pseudogas isnaturally occurring Boron, which as described herein may be easier toobtain and cheaper than enriched Boron. Furthermore, in variousembodiments, the quantity of Boron utilized to create the pseudogas maybe particularly selected based on desired properties of the detector,which may be compared to other neutron detectors.

It should be appreciated that Boron is provided as an example only, andthat in other embodiments, different elements may be utilized to formthe substantially hollow particles. For example, Lithium or Uraniumcontaining particles may also be utilized with embodiments of thepresent disclosure. Utilizing isotopes of Lithium and Uranium, such asLi-6 and U-235, may provide advantage such as avoiding enrichmentprocesses. While the identified isotopes may be relatively rare, asnoted above, eliminating the enrichment processes may be sufficient tocost-effectively develop the substantially hollow particles (e.g., solidparticles). Furthermore, because elemental Uranium is very dense (19.1g/cc), various compounds of Uranium may be utilized. For example, Uranylacetate dehydrate (2.893 g/cc) and Uranyl nitrate hexahydrate (2.81g/cc) may have low enough densities for utilization with embodiments ofthe present disclosure. Additionally, compounds including Cadmium andGadolinium may also be utilized. For example, Cadmium(II) acetate,anhydrous 99% (2.34 g/cc), Cadmium acetate dehydrate (2.01 g/cc), andGadolinium(III) acetate tetrahydrate (1.611 g/cc) may also be potentialcandidates for formulation of the solid particles.

FIG. 1 is a schematic side view of an embodiment of a downhole drillingsystem 10 (e.g., drilling system) that includes a rig 12 and a drillstring 14 coupled to the rig 12. The drill string 14 includes a drillbit 16 at a distal end that may be rotated to engage a formation andform a wellbore 18. In various embodiments, the drill string 14 isformed from one or more tubulars that are mechanically coupled together(e.g., via threads, specialty couplings, or the like). As shown, thewellbore 18 includes a borehole sidewall 20 (e.g., sidewall) and anannulus 22 between the wellbore 18 and the drill string 14. Moreover, abottom hole assembly (BHA) 24 is positioned at the bottom of thewellbore 18. The BHA 24 may include a drill collar 26, stabilizers 28,or the like.

In operation, drilling mud or drilling fluid is pumped through the drillstring 14 and out of the drill bit 16. The drilling mud flows into theannulus 22 and removes cuttings from the face of the drill bit 16.Moreover, the drilling mud may cool the drill bit 16 during drillingoperations and further provide pressure stabilization in the wellbore18. In the illustrated embodiment, the drilling system 10 includes alogging tool 30 that may conduct downhole loggings operations to obtainvarious measurements. The illustration embodiment further includes ameasurement module 32. As will be described below, in variousembodiments, the measurement module 32 may include one or more nuclearsources or detectors for interrogation of the formation. For example,the measurement module 32 may include a neutron or gamma ray (e.g.,gamma) source that emits radioactive energy into the formation. Theradioactive energy may interact with various components of theformation, such as rocks, dirt, hydrocarbon, water, etc. and thereafterfacilitate reactions, such as capture, scattering, and the like. Themeasurement module 32 may further include a radiation detector, whichmay be sufficiently shielded from the radiation source to enabledetection of radioactivity substantially from the formation and not fromthe source. In various embodiments, as will be described below, theradiation detector may be a neutron detector. However, it should beappreciated that gamma detectors, such as spectroscopy detectors, mayalso be used. Furthermore, while the illustrated embodiment includes themeasurement module 32 on the drill string 14, it should be appreciatedthat, in various embodiments, the measurement module 32 may beincorporated into a wireline system, a coiled tubing system, or anyother downhole investigation system.

FIG. 2 is a schematic diagram of an embodiment of a neutron detector 50(e.g., detector), which may be a proportional fill detector orproportional counter. In operation, the detector 50 measures particlesof ionizing radiation. For example, a neutron may enter a chamber filledwith a gas, collide with an atom of the gas, and ionize it to produce anelectron and a positively charged ion. An anode extending through thechamber may be supplied with electrical current, which will increase dueto the electric current flow produced by the resulting electron andpositively charged ion. Detecting the increase in current flow may becorrelated to detection of radiation within the chamber.

In the illustrated embodiment, the neutron detector 50 may be a highpressure Xenon (Xe) detector (e.g., HPXe detector). As will beappreciated, HPXe detectors provide a variety of unique benefits such ashigh stopping power, a low Fano-factor, mechanical and chemicalstability, and low energy for the production of electron-ion pairs.Additionally, HPXe detectors are relatively low cost when compared tocomparable detectors, such as High Purity Germanium (HPGe) and CadmiumZinc Telluride (CZT). Furthermore, HPXe detectors provide thesignificant benefit over HPGe detectors in that HPXe do not use a supplyof cryogenic gas to cool the detector. It should be appreciated that, invarious embodiments, HPXe detectors may be used in gamma rayspectroscopy. However, in various embodiments described herein, the HPXedetector may be modified to enable detection of neutrons. Gamma rayshielding may be provided to selectively transmit impinging neutrons andto reject impinging gamma rays.

As will be described below, the addition of a material, or combinationsof materials, having a high thermal neutron cross section may be addedto the gas chamber to enable detection of thermal neutrons. It should beappreciated that various materials have a high thermal neutron crosssection, and as a result, in various embodiments the material added tothe detector may have a thermal neutron cross section above apre-determined threshold. For simplicity, various embodiments describedherein will use Boron as the material having the high thermal neutroncross section. Accordingly, it should be appreciated that Boron (or invarious embodiments Boron-10) has a thermal neutron cross section atleast greater than the threshold. However, as noted above, otherelements and isotopes may be utilized that have a thermal neutron crosssection greater than the threshold. In various embodiments, Boron (whichincludes a proportion of Boron-10) may be added to the gas chamber toenable detection of thermal neutrons. The illustrated detector 50includes a housing 52, which may act as a cathode during operation. Thehousing 52 encloses a gas chamber 54, which as will be described belowmay include a pseudogas. The pseudogas may be a combination of Xe gasand Boron-containing particles suspended within the Xe gas. Extendingthrough the gas chamber 54 is an anode 56, which receives electricalenergy from a power supply 58. The illustrated embodiment furtherincludes a Frisch grid 60 arranged proximate the anode, but it should beappreciated that the Frisch grid 60 is optional and may be omitted incertain embodiments. It should be appreciated that various features ofthe detector 50 have been omitted for clarity and will not be describedherein, such as various insulators, valves, supports, filters,electrical components, and the like.

Naturally occurring Boron (B) has two different forms, Boron-11 (B-11)and Boron-10 (B-10). B-11 occurs with approximately 80.1 percent naturalabundance, while B-10 occurs with approximately 19.9 percent naturalabundance. B-10 has a substantially higher thermal neutroncross-section, which is directly related to its ability to capturethermal neutrons. That is, a larger neutron cross section is associatedwith a higher likelihood of neutron capture. Accordingly, B-10 ispreferred for neutron detection because the thermal neutron crosssection is greater than the cross section for B-11. Certain radiationdetectors may utilize enriched quantities of B that have a greaterpercentage of B-10 than the naturally occurring levels. However,enriching natural B to high percentage of B-10 may be prohibitivelyexpensive.

Furthermore, as described above, in order to provide the cascadingreaction for detection within the detector 50, particles for interactionwith the ion pairs are required within the gas chamber 54. That is,filling the detector 50 with only B-10 will capture neutrons, but itwill not produce the associated ionizing particles utilized fordetection with proportional gas counters. Some neutron detectors mayinclude Boron Trifluoride (BF₃) consisting of a gas chamber filled withBF₃ gas at approximately 0.5 to 1 atmosphere. BF₃ is a toxic and/orcorrosive gas, and BF₃ detectors utilize high voltage requirements(e.g., 1500 to 2000 volts) to detect neutrons. Furthermore, in variousembodiments of enriched BF₃ may be subject to export controls, furtherincreasing costs because of the costs of compliance.

FIG. 3 is a schematic cross-sectional view of an embodiment of the gaschamber 54 comprising a pseudogas 70 formed at least in part by acombination of a gas 72 and solid particles 74. In various embodiments,the solid particles 74 may be nanoparticles, which are particles havinga diameter between 1 and 100 nm. However, it should be appreciated thatthe particles 74 may not be nanoparticles in various embodiments. Itshould be appreciated that the particle density, dispersion, size, andgeneral arrangement is for illustrative purposes only and is notintended to limit the disclosure. For example, the particle densityand/or ratio of solid particles to gas may be particularly selectedbased on a variety of factors, such as detector size, operatingconditions, costs, and the like. In various embodiments, the solidparticles 74 may be Boron-containing and also substantially hollow,spherical particles that “float” or are otherwise suspended within thegas chamber 54. However, as noted herein, the substantially hollow solidparticles 74 may contain other elements and/or isotopes. One method ofproduction of Boron-containing nanoparticles 74 is discussed in“Controlled Fabrication of Ultrathin-shell BN Hollow Spheres withExcellent Performance in Hydrogen Storage and Wastewater Treatment” byLian et al. in Energy & Environmental Science, Issue 5, 2012, which ishereby incorporated by reference. This paper describes 50 nm diameterBoron Nitride microspheres having 2 nm wall thickness so that only 22%of the sphere's total volume is Boron Nitride, which, if solidly-packed,would have 1900 times the Boron per unit volume as the typical BF₃detector. By way of example, the density of solid Boron Nitride is 2.100g/cc, so the density of these hollow Boron Nitride microspheres is 0.465g/cc, which, according to the online NIST Chemistry WebBook, equals thedensity of Xe gas at 783 psi and 25 C or the density of Ar gas at 4120psi and 25 C. A solid Boron Nitride particle would float in Xe gas at2800 psi and 25 C or in Ar gas at greater than 145,000 psi and 25 C. Thedensity of pure Boron is 2.46 g/cc so it would float in Xe gas at 7300psi and 25 C. In various embodiments, particles of low toxicity and lowenough mass density (e.g., below a threshold amount) enable gaspressures within reasonable values while still maximizing a number ofmoles of Boron per unit volume.

The Boron molar density (mol/cc) is the compound's molar densitymultiplied by the number of Boron atoms in that compound. A compound'smolar density (mol/cc) is its mass density (g/cc) divided by itsmolecular weight (g/mol). The properties of Boron, B, are 2.46 g/cc,10.811 g/mol, with 0.228 mol/cc of Boron. The properties of BoronCarbide, B₄C, are 2.52 g/cc, 55.255 g/mol, with 0.182 mol/cc of Boron.The properties of Boron Nitride, BN, are 2.10 g/cc, 24.817 g/mol, with0.085 mol/cc of Boron. The properties of Boron Oxide, B₂O₃, are 2.46g/cc, 69.618 g/mol, with 0.071 mol/cc of Boron. Boron Carbide and BoronNitride have low toxicity but Boron Carbide has the higher molar densityof Boron. Hollow spheres of Boron Carbide have been made (J. L. Wang etal., “Boron Carbide Hollow Microspheres Prepared by Polymer DerivedMethod”, Key Engineering Materials, Vol. 726, pp. 159-163, 2017), whichare 8.7% solid corresponding to a density of 0.22 g/cc, which wouldfloat in Xe at 480 psi at 25 C or in Ar at 1880 psi at 25 C. If thewalls of these hollow Boron Carbon spheres were made thicker to equal22% of the sphere's volume (as was the case for the earlier example ofBoron Nitride hollow spheres) then the density would be 0.56 g/cccorresponding to Xe at 845 psi and 25 C and a solidly-packed detectorusing such Boron Carbide hollow particles would have approximately 4090times more B-10 than an equivalent BF₃ detector.

As used herein, the pseudogas 70 refers to a mixture of gas atoms 72 andsolid particles 74, such as the Boron-containing particles. In certainembodiments, properties of the gas atoms 72 and particles 74 may beparticularly selected to suspend the particles 74 among the gas atoms72. In various embodiments, the illustrated gas atoms 72 are Xe atoms,but it should be appreciated that other gases, such as Ar and the like,may be utilized. Xe gas provides the advantage of being non-radioactive,having the highest density of any non-radioactive inert gas, andproviding ample atoms for ionization in a proportional gas tube.

In the illustrated embodiment, the Boron-containing particles 74 may beformed such that they have substantially neutral buoyancy relative tothe Xe gas 72. That is, the Boron-containing particles 74 may be said to“float” or otherwise settle very slowly within the gas chamber 54, forexample over a period of several days or longer. In other words, thedensity of the Boron-containing particles 74 may be calculated to beapproximately equal to the density of the Xe gas 72. Once filled, thevolume of the chamber is fixed, which means that the number of moles oftrapped gas is fixed and the corresponding mass density of the trappedgas is also fixed. Any increase in temperature will increase thepressure but it will not change the mass density. As will beappreciated, the mass density of the Boron-containing particles 74 maybe calculated to be substantially equal to the density of the Xe suchthat the Boron-containing particles 74 will have neutral buoyancy withinthe gas chamber 54.

The fraction of solid material in the hollow (or substantially hollow)particles may be calculated from their diameter and wall thickness.Assuming negligible mass density of any gas that may have been trappedinside of the hollow during fabrication, the hollow particle's massdensity is the solid fraction multiplied by the mass density of thatsolid. The number of B-10 nuclei per unit volume may be calculated fromthe total number of B nuclei per unit volume multiplied by the 19.9%natural abundance of B-10. In this manner, the volume of theBoron-containing particles 74 may be calculated to provide formanufacturing and filling of the gas chamber 54.

As described above, BF₃ detectors may be utilized despite the variousdrawbacks. Accordingly, when designing embodiments of the presentdisclosure, various properties of B within the BF₃ detector may beevaluated. For example, BF₃ detectors may have a particular weightfraction of B-10 or B-10 mole density. Accordingly, the same values forthe Boron-containing particles 74 may be calculated and compared to BF₃detectors. In various embodiments, utilizing Boron-containing particles74 may enable significantly more B-10 within the gas chamber 54 than byutilizing BF₃ detectors. Furthermore, the Boron-containing particles 74may be formed from naturally occurring B, rather than enriched B-10,which decreases the cost. In various embodiments, a solidly-packeddetector using the Boron-containing particles 74 may have approximately1900 times more B-10 than an equivalent BF₃ detector. As such, thedetector 50 of the illustrated embodiment need not be solidly-packed tohave adequate B-10 for interaction with neutrons. However, as describedabove, it should be appreciated that other configurations ofBoron-containing particles 74 may have approximately 4000 times moreB-10 than an equivalent BF₃ detector.

As described above, in various embodiments the gas chamber 54 includesthe pseudogas 70 comprising a mixture of both the Xe atoms 72 (e.g., Xegas, gas) and the Boron-containing particles 74. The Boron-containingparticles 74 may be dispersed throughout the gas chamber 54, and neednot “fully pack” the gas chamber 54 so as to enable sufficient Xe gasparticles 74 for interaction with the ion pairs generated throughneutron capture. Accordingly, the Boron-containing particles 74 may bedispersed throughout the Xe gas particles 72 within the gas chamber 54,as shown in the embodiment illustrated in FIG. 3. In variousembodiments, the gas chamber 54 may be pressurized to a particularpressure, such as approximately 1000 psi.

It should be appreciated that, when the Boron-containing particles arenanoparticles, then, due to their small size and weight, the settlingand collection of the Boron nanoparticles 74 may be significantlyreduced or adjusted for. Because particle settling under gravity is afunction of aerodynamic size, very small particles (e.g., AerodynamicEquivalent Diameter, AED, less than 100 nm) can take up to one month tosettle out of completely still air. Additionally, gas density may alsoinfluence settling time. Accordingly, settling within the gas chamber 54may be inconsequential for anticipated operations, which are usuallycompleted within a few days. Furthermore, it should be appreciated thatthe long settling time enables the detector 50 to be remixedautomatically by shakes or external forces due to tripping into and outof the wellbore.

As noted above, Boron-containing particles are described as an example,but embodiments of the present disclosure are not limited toBoron-containing particles. For example, the particles may be formedfrom other isotopes, such as Li-6 and/or U-235 and/or compounds thereof.For example, light-weight compounds of Lithium and Uranium may beutilized due to the maximum practical density of Xenon beingapproximately 2.5 g/cc. As noted above, Uranyl acetate dehydrate (2.893g/cc) and Uranyl nitrate hexahydrate (2.81 g/cc) may be utilized inembodiments of the present disclosure. Other non-limiting examplesinclude LiF (2.635 g/cc), Li₂CO₃ (2.11 g/cc), and Li₃PO₄ (2.40 g/cc).Furthermore, lightweight compounds of Cadmium and/or Gadolinium may alsobe used, including as non-limiting examples Cadmium(II) acetate,anhydrous 99% (2.34 g/cc), Cadmium acetate dehydrate (2.01 g/cc), andGadolinium(III) acetate tetrahydrate (1.611 g/cc).

FIG. 4 is a schematic cross-sectional view of an embodiment of the gaschamber 54 comprising the pseudogas 70. In the illustrated embodiment, aneutron particle 80 interacts with a Boron-containing particle 74 (whichmay be a particle of any element having a thermal neutron cross sectionabove the threshold) to produce an alpha particle 82. It should beappreciated that a Lithium-7 particle may also be produced due to thecapture of the neutron particle 80, but this has been omitted forclarity in the discussion. The energy from the alpha particle 82interacts with the Xe particles 72 to energize the anode 56, therebyenabling detection of the neutron capture within the detector 50. Asdescribed above, by reducing the packing of the Boron-containingparticles within the gas chamber 54, additional Xe particles 72 may beincluded, thereby increasing the available particles for ionization bythe release of the alpha particle 82. Accordingly, the detector 50 maybe utilized to detect neutrons with reduced costs compared totraditional detectors, such as He-3 or BF₃ because the illustrateddetector 50 uses solid particles that contain Boron and, generally, thenumber of moles per unit volume of an element that is part of a solid isfar greater than the number of moles per unit volume of that sameelement if it had existed in the form of a gas. Because the molardensity of Boron in a solid is so high compared to a gas, it becomespossible to use unenriched, naturally-occurring Boron, which is lesscostly than both He-3 and enriched B-10.

FIG. 5 is a flow chart of an embodiment of a method 100 for preparing aneutron detector. It should be understood that, for any process ormethod described herein, that there can be additional, alternative, orfewer steps performed in similar or alternative orders, or concurrently,within the scope of the various embodiments unless otherwisespecifically stated. The method 100 may begin by forming a gas chamberhaving at least an anode and cathode (block 102), such as the gaschamber 54, anode 56, and the housing 52 (which may act as a cathode).In other words, a proportional gas counter may be formed using variousmaterials, as will be appreciated by one skilled in the art. The methodmay continue by calculating the desired boron ratio within the gaschamber (block 104). For example, as described above, the gas chambermay be compared to an equivalent BF₃ detector and thereafter a ratiobetween the two may be calculated. In various embodiments, the ratio ofa gas chamber packed solidly to maximum number of Boron-containingparticles may be significantly greater than the equivalent BF₃ detector,thereby enabling the gas chamber of the embodiments herein to not needto be packed solid with Boron-containing particles and, thereby, providemore than ample space for ionization of the gas in the chamber. Whilethe method is described with respect to Boron-containing particles, asnoted herein, other elements and isotopes may be utilized. As a result,the calculation may be with respect to other isotopes, such as Li-6,U-235, Cd-113, Gd-157. Thereafter, the Boron-containing particles (orother particles) may be formed (block 106). The gas chamber is thenfilled with a gas, such as Xenon, and the Boron-containing particles (orother particles) (block 108). In various embodiments, the size of theBoron-containing particles may be small enough that settling may not besignificant over short periods of time, although neutral buoyancy may beobtained in various embodiments to insure that that distribution ofsuspended solid particles is uniform or substantially uniform in thechamber. Furthermore, a calculated density of the Boron-containingparticles may be substantially equal to the gas, thereby enabling thenanoparticles to “float” or otherwise be suspended within the gas togenerate a pseudogas. These desirable characteristics may also becalculated and formed with respect to other isotopes utilized withembodiments of the present disclosure. Next, the gas chamber is sealed(block 110) to form the gas detector.

FIG. 6 is a flow chart of a method 120 for performing downholeoperations using a gas detector. In the illustrated embodiment, themethod 120 begins with forming a pseudogas gas comprising gas and solidparticles (block 122). In various embodiments, the pseudogas may beformed between Xe gas and Boron-containing particles, which in variousembodiments may be hollow, but other isotopes may be utilized. It shouldbe appreciated that the density of the Boron-containing particles may besubstantially equal to the density of the Xe gas such that theBoron-containing particles are substantially suspended or “float” withinthe Xe gas. In various embodiments, the Boron-containing particles maybe have neutral buoyancy within the Xe gas. Next, a proportional gascounter is formed using the pseudogas (block 124). The detector may bearranged on a drill string for conducting downhole operations, such asdrilling, logging, measurements, and the like (block 126). The drillstring may be lowered into a wellbore (block 128) and the formation maybe interrogated with radioactive energy, in certain embodiments.Thereafter, the detector may be used to detect neutron particles emittedfrom the formation. It should be appreciated that, in other embodiments,the formation may not be interrogated with radioactive energy. Thedetector may receive information (block 130) corresponding to theneutron activity of the formation. This information may be utilized toanalyze and determine various properties of the formation, such as thecomposition of the formation. In this manner, downhole operations may beperformed using a gas detector including a pseudogas.

While various embodiments of the instant application may be directed tooil and gas technologies, for example formation evaluation, it should beappreciated that embodiments of the present disclosure may be deployedin a variety of industries and technology sectors. For example, securityand defense industries may utilize embodiments of the present disclosurefor screening purposes, for example at international borders, transitcenters, shipping areas, and the like. Additionally, in variousembodiments, medical devices such as PET/CT and SPECT/MRI instrumentsmay utilize neutron detector technologies. Moreover, scientific researchapplications, such as neutron scatting beam lines, may benefit from theuse of the disclosed neutron detectors. Other potential applicationsalso include industrial monitoring (e.g., personnel, soil, water, etc.),nuclear decommission and inspection, aerospace, and energy.

The foregoing disclosure and description of the disclosed embodiments isillustrative and explanatory of the embodiments of the invention.Various changes in the details of the illustrated embodiments can bemade within the scope of the appended claims without departing from thetrue spirit of the disclosure. The embodiments of the present disclosureshould only be limited by the following claims and their legalequivalents.

The invention claimed is:
 1. A system for detecting neutrons, the systemcomprising: a housing; a gas chamber at least partially defined by thehousing; an anode extending through at least a portion of the gaschamber; and a pseudogas arranged within the gas chamber, wherein thepseudogas comprises a mixture of a gas and solid particles, the solidparticles containing an element that generates a charged particle afterabsorbing a thermal neutron.
 2. The system of claim 1, wherein the gasis a dense gas.
 3. The system of claim 1, wherein the solid particlescontain at least one of Li-6, U-235, Cd-133, or Gd-157.
 4. The system ofclaim 1, wherein a density of the gas is substantially equal to adensity of the solid particles such that the solid particles aresuspended within the gas.
 5. The system of claim 1, wherein the housingis a cathode and the anode detects a current change in the anode due atleast in part to neutron capture by the solid particles.
 6. The systemof claim 1, wherein the solid particles are formed from a materialhaving a thermal neutron cross section greater than a threshold amount.7. The system of claim 1, wherein the proportion of solid particles ofthe pseudogas does not fully fill the gas chamber.
 8. The system ofclaim 1, wherein the solid particles are formed from Uranium-containingparticles, and the Uranium-containing particles are a compound.
 9. Thesystem of claim 1, wherein the solid particle are formed fromLithium-containing particles, and the Lithium-containing particles are acompound.
 10. The system of claim 1, wherein the solid particles areformed from Cadmium-containing particles, and the Cadmium-containingparticles are a compound.
 11. The system of claim 1, wherein the solidparticle are formed from Gadolinium-containing particles, and theGadolinium-containing particles are a compound.
 12. The system of claim1, wherein the element includes at least one of a Lithium-containingparticle, a Uranium-containing particle, a Cadmium-containing particle,or a Gadolinium-containing particle that is naturally occurring.
 13. Asystem for detecting neutrons, the system comprising: a housing forminga cathode of a proportional gas counter; a gas chamber formed at leastpartially by the housing; an anode extending partially through the gaschamber; and a pseudogas formed from a combination of dense gas andLithium-containing particles positioned within the gas chamber, whereinthe Lithium-containing particles are arranged to capture incomingneutrons and increase a current at the anode via the production ofcharged particles from neutron capture.
 14. The system of claim 13,wherein the Lithium-containing particles are formed as hollow spheresfrom naturally occurring Lithium-containing compounds.
 15. The system ofclaim 13, wherein a density of the dense gas is substantially equal to adensity of the Lithium-containing particles.
 16. The system of claim 13,wherein a diameter of the Lithium-containing particles is below athreshold amount to reduce a settling time of the Lithium-containingparticles.
 17. The system of claim 13, wherein the housing is arrangedin a downhole formation.
 18. A system for detecting neutrons, the systemcomprising: a housing forming a cathode of a proportional gas counter; agas chamber formed at least partially by the housing; an anode extendingpartially through the gas chamber; and a pseudogas formed from acombination of dense gas and Uranium-containing particles positionedwithin the gas chamber, wherein the Uranium-containing particles arearranged to capture incoming neutrons and increase a current at theanode via the production of charged particles from neutron capture. 19.The system of claim 18, wherein the Uranium-containing particles areformed as hollow spheres from naturally occurring Uranium-containingcompounds.
 20. The system of claim 18, wherein a density of the densegas is substantially equal to a density of the Uranium-containingparticles.